1. Field of the Invention
The subject matter of the present invention relates to a device for the continuous filtration of material blends, in particular for separating impurities from plastic melts.
2. Prior Art
Used plastics and plastic waste normally contain high quantities of foreign substances, e.g., metal parts, paper residues, glass, secondary plastics, etc. As a rule, these foreign substances or impurities must be removed prior to the recycling of the plastics. In many cases, this is accomplished by first plastifying the used plastic materials by heating them and by subsequently filtering the plastic melt. For this purpose, so-called melt filtration units are used, by means of which the foreign metal or nonmetal substances or higher-melting plastics are separated. To make possible a continuous and troubleproof filtration, however, the melt filtration units must be continuously cleaned.
German Patent No. DE 43 08 685 C2 describes a generic separating unit in which the contaminated plastic melt is pressed into the inside chamber of a filtration body having the shape of a hollow cylinder and being disposed in a housing. Coaxially disposed thereto in the inside chamber of the filtration body is a rotary-drive scraper shaft which, together with the inside wall of the filtration body, defines an inside annular chamber and which, along its outside, has a plurality of scrapers that run at an oblique angle with respect to the direction of the axis and which together constitute a helical screw. The residues retained by the filtration body on its inside surface are scraped off by the scrapers and, through the rotation of the scraper shaft, are transported to a material outlet point along the filtration body, which material discharge point is located axially opposite the inlet end of the inside annular chamber. In this embodiment, the transporting path for discharging the residues, however, extends across the entire inside surface of the filter, which can quickly put considerable strain on the filtration body, especially if the residues are hard and abrasive. Furthermore, because of the axial movement of the residues along the filter, the filter openings can easily become clogged. In addition, the filtration body of this known separating unit is supported on the housing by means of a constructively complex supporting unit.
German Patent No. DE 32 39 030 A1 describes another separating unit for recycling mixtures of thermoplastic materials and material waste. In this case, the inside chamber of a filtration body in the shape of a hollow cylinder also carries a rotary-drive scraper shaft, on the outside circumference of which scrapers are disposed so as to be staggered with respect to one another both in the axial and in the circumferential direction. Disposed in the filtration body are oppositely placed slotlike outlet openings to which the residues that accumulate on the inside surface of the filtration body are transported by the scrapers as a result of the reciprocating rotary movement of the scraper shaft. However, in this case, the residues being transported to the two outlet openings are again moved for a relatively long time along the inside wall of the filter.
Thus, the problem to be solved by the present invention is to make available a device of the type mentioned in the introduction which makes it possible for the separated residues to be removed from the surface of the filter as fast as possible.